1. Field of the Invention
The present invention is related to an optical scanning apparatus using a MEMS mirror as a deflection means and an image forming apparatus such as a copy machine and a printer provided with the optical scanning apparatus.
2. Related Art
In an image forming apparatus such as a copy machine and a printer, a surface of an image carrying body is uniformly charged by a charging device, the image carrying body is exposed and scanned by an optical scanning apparatus, and an electrostatic latent image is formed on the surface of the image carrying body corresponding to image information. Thereafter, the electrostatic latent image is developed by a developing device using a toner as a developer, and visualized as a toner image. The toner image is transferred onto paper by a transfer device, and then heated, pressurized and fixed onto the paper by a fixing device. Ejection of the paper onto which the toner image is fixed terminates a series of image forming operations.
In an optical scanning apparatus, a polygon mirror and a galvano mirror have been mainly used conventionally as a deflector for scanning an optical beam. However, in order to achieve higher resolution image and higher speed printing, it is required to rotate the polygon mirror and the galvano mirror at higher speed.
Since high speed rotation of the polygon mirror and the galvano mirror may produce problems related to bearing durability, and heat and noise due to windage (energy loss due to air resistance during rotation), there is limit in the speeding up of scanning.
Given this, in recent years, a deflector using silicon micromachining technology (Micro Electro Mechanical Systems, MEMS) is being developed. For example, a method is proposed in which a micro mirror (hereinafter referred to as MEMS mirror) and a torsion beam for pivotally supporting thereof are integrally formed on a Si substrate and AC voltage is applied between a movable electrode on a MEMS mirror side and a fixed electrode on a fixed side, thereby torsioning the torsion beam by an electrostatic attractive force between the electrodes and oscillating the MEMS mirror using resonance (for example, see Japanese Unexamined Patent Application Publication No. H04-211218).
According to the abovementioned method, due to oscillation (sinusoidal vibration) of the MEMS mirror using resonance, the effects of providing high-speed movement and suppressing noise and power consumption can be obtained. On the other hand, the MEMS mirror has a smaller deflection angle than that of the polygon mirror and size of reflection surface of the MEMS mirror is limited.
However, the MEMS mirrors using resonance of Si substrate have different deflection angles due to variable inherent resonance frequency. In addition, deflection angle characteristics thereof vary according to change in temperature and barometric pressure, and thus scanning performance such as linearity varies according to environmental variation. Therefore, a technology for compensating the deflection angle is required that, in a case in which a maximum deflection angle of a MEMS mirror is smaller than a predetermined value, approximates the deflection angle to the predetermined angle by increasing voltage supplied to an electrode of the MEMS mirror. In addition, a technology for monitoring the maximum deflection angle, or a scanning speed, which is an equivalent parameter.
Regarding the technology for monitoring the maximum deflection angle or the scanning speed of MEMS mirror, for example, Japanese Unexamined Patent Application Publication No. H1-100509 proposes a technology of measuring time interval between two signals generated when a resonance scanner goes back and force above a scanning position sensor disposed outside a scanned region and controlling drive voltage of the resonance scanner such that the time interval becomes equal to a predetermined value.
In addition, Japanese Unexamined Patent Application Publication No. H3-134613 proposes a technology of controlling a torsion angle of a reflection mirror according to a detection result of a temperature detection means. Japanese Unexamined Patent Application Publication No. 2004-029064 proposes a technology of measuring induced current generated in an internal coil of a MEMS mirror and monitoring a deflection state of the MEMS mirror based on a result thereof. Furthermore, Japanese Unexamined Patent Application Publication No. 2006-047590 proposes a technology of calculating a scanning speed based on an interval between detection signals from detectors provided in the vicinity of both end portion of a scanning range of an optical beam.
An MEMS mirror changes in deflection angle characteristics according to change in temperature and barometric pressure. Therefore, in a case of using such a MEMS mirror in an optical scanning apparatus for an image forming apparatus, scanning performance such as linearity varies according to environmental variation.
In addition, if a driving frequency is not approximate to a resonance frequency (natural frequency) of the MEMS mirror, the maximum deflection angle of the MEMS mirror is greatly decreased. Therefore, the driving frequency must be set to a value that is approximate to the resonance frequency of the MEMS mirror.
However, MEMS mirrors have variable resonance frequencies (natural frequencies) due to minute differences in manufacture. As a result, if the MEMS mirrors are driven at the same driving frequency, the respective maximum deflection angles thereof are different. In other words, the MEMS mirrors with different maximum deflection angles provide variable scanning performance. Such a MEMS mirror used in a scanning apparatus for an image forming apparatus causes a problem of deteriorated image.
Given this, for example, Japanese Unexamined Patent Application Publication No. H1-100509 proposes a technology of measuring time interval between two signals generated when a resonance scanner goes back and force above a scanning position sensor disposed outside a scanned region and controlling drive voltage of the resonance scanner such that the time interval becomes equal to a predetermined value.
In addition Japanese Unexamined Patent Application Publication No. 2009-031671 proposes a technology of reducing image deterioration such as jitter by modulating a clock that determines timing of blinking of a light source. Japanese Unexamined Patent Application Publication No. 2005-195869 proposes a technology of compensating the maximum deflection angle of a MEMS mirror to be a uniform value by changing a driving frequency. Japanese Unexamined Patent Application Publication No. 2007-086626 proposes a technology of, when an interval between detection signals output from a photodetector device is changed due to change in a torsion angle of a MEMS mirror, compensating the torsion angle of the MEMS mirror to be uniform by adjusting applied voltage to the MEMS mirror based on the change of the interval.